Frankenstein monster is kinda like Trans folks or people

[Sheldon]

What objective evidence do we have that an individual's gender differs from their biological sex? 

I've answered this more than once already, so I am not sure why you keep repeating the question?

What can we do to objectively verify that pain exists other than this? (An fMRI shows that some activity is occurring, but does not tell us what the experience is like for the person having the experience.)

As I said, the experience of pain is subjective, it's existence is however can be supported by objective evidence. Not unlike gender dysphoria then.

"Pain is often accompanied by quantifiable physiological parameters, such as increased heart rate, blood pressure, and stress hormones."

"Skin conductance increases when the autonomic nervous system is activated during pain."

"Functional magnetic resonance imaging (fMRI) and electroencephalograph (EEG) can visualize brain activity that is modulated by analgesics."

"Dynamic pain connectome analysis can identify alterations in neural network connectivity that correlate with pain severity."

"Medical tests, such as diagnoses, assessment procedures, and certifications, can objectify pain and validate its existence."

"Behavioral responses and verbal communication can be observed and measured."

The experience is subjective, but there is objective evidence that pain exists.